An oxygen concentration system, particularly a pressure swing adsorption (PSA) oxygen concentration system, is used to provide an output gas mixture which has a higher concentration of oxygen than is ordinarily found in air. Specifically, the pressure swing adsorption oxygen concentration system separates oxygen and argon from the remainder of the gases, such as nitrogen, found in air, and produces an output gas mixture which is ideally 95.6% oxygen and 4.4% argon. In actual practice, however, an oxygen concentration system usually produces an output gas mixture consisting of about 90% to 93% oxygen. A 2% to 5% constituency of nitrogen in the output gas mixture of the oxygen concentration system is generally acceptable for many industrial and medical applications.
A typical pressure swing adsorption oxygen concentration system includes two desiccant towers each having a desiccant bed. While a first of these two desiccant towers is operated in an adsorption cycle such that its desiccant bed adsorbs moisture and nitrogen from an inlet air in order to produce a dry output gas mixture which is rich in oxygen, the second of the two desiccant towers is operated in a purge cycle. During this purge cycle, moisture and nitrogen are purged from the desiccant bed of the second desiccant tower. Both of the desiccant towers in a pressure swing adsorption oxygen concentration system need a substantial volume of unsaturated desiccant material above the mass transfer zone in order to ensure adequate adsorption of nitrogen. With an adequate volume of unsaturated desiccant material above the mass transfer zone, normal variations in the temperature, humidity, and pressure of the inlet air will not significantly affect the concentration of oxygen in the output gas mixture. Accordingly, stable operation of the pressure swing adsorption oxygen concentration system is achieved.
A relatively new application that is possible for the pressure swing adsorption oxygen concentration system is the production of feed gas for the generation of ozone (0.sub.3). Ozone is used, for example, in the purification of water and in the treatment of sewage. Ozone may be typically produced by a corona discharge ozone generator. Recent studies indicate that a corona discharge ozone generator achieves optimum operation when supplied with a feed gas having a concentration of oxygen between 50% and 65%. The other gases in the optimum feed gas supplied to the corona discharge ozone generator are inert gases such as nitrogen and argon.
While it might be possible to use the pressure swing adsorption oxygen concentration system, which produces an output gas mixture having approximately a 90% concentration of oxygen, in order to supply feed gas to a corona discharge ozone generator, which requires a feed gas having a concentration of oxygen between 50% and 65%, such a use of a pressure swing adsorption oxygen concentration system is not straightforward. For example, one way to achieve a reduced concentration of oxygen in the output gas mixture of a pressure swing adsorption oxygen concentration system is to increase the flow of gas through its desiccant towers. Thus, if the outlet flow from a pressure swing adsorption oxygen concentration system exceeds its design limit, the concentration of oxygen in its output gas mixture decreases since all of the nitrogen can no longer be adsorbed from the inlet air supplied to the pressure swing adsorption oxygen concentration system. By so overloading the pressure swing adsorption oxygen concentration system, the concentration of oxygen in its output gas mixture can theoretically be reduced to the desired 50% to 65% range. However, there would be little or no unsaturated desiccant material above the mass transfer zone if the gas flow through the desiccant towers is increased to such an extent. As a result, any slight variations in the temperature, humidity and/or pressure of the inlet air would cause radical changes in the concentration of oxygen in the output gas mixture, and could cause degradation of the desiccant beds to such an extent that a system failure would result. Accordingly, as a practical matter, a pressure swing adsorption oxygen concentration system cannot achieve a stable operating condition when the flow of gas through its desiccant towers is increased sufficiently to produce the desired concentration of oxygen in the output gas mixture.
Another approach to achieve the desired 50% to 65% concentration of oxygen in the output gas mixture is to operate the pressure swing adsorption oxygen concentration system optimally so as to produce the typical 90% concentration of oxygen in its output gas, and then mix this output gas with dry air in the proper proportion to obtain the required 50% to 65% concentration of oxygen in the output gas mixture. However, the dry air to be mixed with the output gas of the pressure swing adsorption oxygen concentration system must be compressed to at least the same pressure as the output gas from the pressure swing adsorption oxygen concentration system and must be dried to at least -60.degree. C. atmospheric dew point in order to meet the dew point requirements for a corona discharge ozone generator. This arrangement requires both a pressure swing adsorption oxygen concentration system and an air drier wherein the air drier delivers a dry gas at or below the required dew point to the output gas of the pressure swing adsorption oxygen concentration system.